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Would an antimatter apple fall up?

New experiments are being proposed to test a big unknown in physics&colon; how antimatter reacts to gravity.

Physicists have studied antimatter, the mirror version of ordinary matter, for decades. They know, for example, that antiparticles have the same mass as ordinary particles, but opposite charge. But no one knows what effect gravity will have on such particles.

Now several groups want to measure exactly how the Earth will pull on antimatter. The tests would create a horizontal beam of the stuff and measure how much gravity deflects it.

The complicated ballistic test may show no difference between the way matter and antimatter fall. But some experimentalists are holding out hope that they may see something completely unexpected, which could point the way to new gravity-like forces, or perhaps even antigravity.

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“If antimatter fell down faster, it would mean the discovery of at least one new force, probably two. If it fell up, it would mean our understanding of general relativity is incorrect,” says Thomas Phillips, a physicist at Duke University in Durham, North Carolina, US.

Repulsive force

Gravity is largely expected to have the same effect on antimatter as it does on matter. But theories of quantum gravity, which attempt to unite quantum mechanics and general relativity, allow for the possibility of two other gravity-like forces.

In ordinary matter, these forces would oppose each other, cancelling out any effect. For antimatter, these two forces could add together, pulling such particles towards Earth even harder.

As for the possibility of antigravity, it is not ruled out in the standard model of physics. As a result, some researchers have suggested antimatter may be repelled by gravity. That could explain why so little antimatter is found today, even though theories predict it should have been as plentiful as matter in the early universe.

New physics?

This possibility of observing strange new physics has led several researchers, including Phillips, to lobby for new accelerator experiments. Phillips is proposing to test antihydrogen for such behavior at Fermilab in Batavia, Illinois, US.

At least two other proposals are in the works for CERN in Geneva, Switzerland. The more developed proposal is for an experiment called AEGIS, which would be set up at CERN’s Antiproton Decelerator and if approved, might be able to return its first data within five years.

Each experiment would test the effect of gravity on antihydrogen, particles which have the same mass as hydrogen but contain antiprotons instead of protons and positrons instead of electrons.

There are multiple ways to make antihydrogen. All of them begin with antiprotons, which are made in accelerators by smashing protons into a stationary target.

‘No cup of tea’

But antihydrogen is difficult to control. And since it annihilates on contact with matter, it might be difficult to pinpoint its location using detectors made of normal matter, says Michael Nieto, a theoretical physicist at Los Alamos National Laboratory in New Mexico, US.

“That experiment is hard, and I applaud them for having the guts to try it,” Nieto said of the AEGIS proposal. “This is not like most particle physics experiments, where you know what the beam is, you know how to handle it, and you know the detector will work. It’s no cup of tea.”

AEGIS aims to measure the position of the antihydrogen beam by sending it through two venetian blind-like diffraction gratings. As the beam goes through the first grating, some antihydrogen would annihilate with the grating itself, while the rest would pass through to the next grating, where the same thing would happen again.

The resulting light-dark patterns could pinpoint the position of the beam with 1% accuracy, the AEGIS team says. But Nieto told New Scientist&colon; “If they got 10%, I would just be aghast and cheering wildly.”

Leaning tower

The AEGIS team acknowledges that the experiments are tricky and might show no difference between matter and antimatter, while Phillips says any differences in gravity’s effect on antimatter might be too small to detect. But they say the projects are worth the effort.

“If I had to bet a case of champagne, I would bet that antihydrogen and hydrogen fall exactly the same,” AEGIS project member Michael Doser told New Scientist. “And that’s a case of champagne I’d love to lose – we’re dreaming to see something unexpected.”

Previous experiments have been attempted to test the effect of gravity in charged particles of antimatter. In the early 1990’s, a sub-atomic replication of Galileo’s Leaning Tower of Pisa experiment was set up at CERN to test how quickly protons and antiprotons fall.

Because the particles are charged, they are very sensitive to any stray electric or magnetic field. Project lead Michael Holzschieter of the University of New Mexico in Albuquerque, US, says the test, which boasted a set-up that was simpler than the newly proposed antihydrogen experiments, might have worked, with similar precision to AEGIS. But in 1995 the experiment was shut down early due to the closure of the team’s antiproton source.